The FBI and U.S. Marshals Service are now involved in the search for a missing Waukesha County teen.

"She's been gone a few days. She's a minor, so we treat that very seriously, and that's why we're bringing in all resources to bear. Not just from FBI in Wisconsin, but national assets that are coming in today and tonight. We are assisting the inspector with the sheriff's office," FBI Special Agent in Charge Chris Combs said.

Kathryn Stalbaum, 15, texted a friend Tuesday morning that she would ride her bicycle to school after missing the bus. The teen has not been seen or heard from since.

"We don't have any immediate signs of foul play," he said. "We don't know what happened to Kathryn. We don't know if she left voluntarily. There's a lot we don't know, so we are not sure which details are important here. So I don't want to say the wrong thing, so we're really being very careful."

WISN 12 News has learned detectives traveled to Minneapolis to talk face-to-face with Stalbaum's adult sister there, who said she doesn't know where Kathryn is.

Investigators said they have been in regular contact with Stalbaum's biological parents.

Severson said Friday that Crime Stoppers of Waukesha County and the Waukesha Deputy Sheriff Association are offering a reward of up to $1,000 for information that leads to the safe return of Stalbaum.

Stalbaum is 5 feet 7 inches tall, has long brown hair and braces. She was last seen wearing a sweatshirt, jeans and pink Nike shoes.

Anyone with information about Stalbaum is asked to call the Waukesha County Sheriff's Department at 262-446-5090. Tips can also be submitted online at www.stopcrimewaukesha.com.

On Thursday, about three dozen firefighters joined Waukesha County sheriff's deputies in searching a roughly 4-square-mile area of the town of Genesee.

"It is out of character for her. She has a good relationship with her family. She's a communicative person who keeps her family apprised of what she's doing, so it is out of character for her to be out without having regular contact. She has not been a runaway before," Severson said.

The search included dogs and moving a dive team from a practice to a search of a nearby pond.

"We are pretty much on standby for whatever they need right now," Genesee Fire Department Chief Jim Moon said.

"So you're done searching?" WISN 12 News reporter Nick Bohr asked.

"We're done searching this section for now. They may come up with a different area to search at a later date. There's nothing right now that's going to change out here," Moon said.

But they are continuing to ask people to keep a lookout for Stalbaum.

"Just keep your eyes open -- and pray, I guess, would be the best thing," Moon said.

"This is a difficult case for us. We're really at a loss," Severson said.

Waukesha County authorities are checking social media and whether the Internet might provide any clues, but didn't want to comment on any exact details.

"We have collected videotapes from some businesses, so we are obviously looking over all those videos very carefully to see if we have any images of Kathryn," Severson said.

Investigators said if Stalbaum did leave on her own, they have a message for her.

"The message to Kathryn is that her family is very anxious to see her come home. She is in no trouble whatsoever. If she feels she is in trouble, it's nothing we can't work out," Severson said.

MORE NEWS ON THE WAY TRYING TO HELP FIND AND MISSING WAUKESHA COUNTY TEEN. THE FBI IS NOW INVOLVED IN THIS INVESTIGATION. THE FULL FORCE OF THE FBI -- THE INSPECTOR FOR THE WAUKESHA COUNTY DEPARTMENT TOLD ME THAT HE STILL DOES NOT KNOW WHETHER SHE WAS ABDUCTED OR SHE HAS RUN AWAY. THE MOST SIGNIFICANT THING SHE DID WAS RELEASE OF THESE NEW PHOTOS. WE HAVE THREE OF THEM THAT WE HAVE GIVEN -- THEY HAVE GIVEN US NOW. THEY WANT PEOPLE TO DOWNLOAD THESE PHOTOGRAPHS FROM THE DEPARTMENT FACEBOOK PAGE ONTO THEIR SMARTPHONES AND HAD THEM ON THE READY IF THEY THINK THEY HAVE SPOTTED THE MISSING TEENAGER KATHRYN STALBAUM. SHE HAS BEEN MISSING SINCE TUESDAY MORNING. SHE MISSED THE SCHOOL BUS AND TESTING A FRIEND -- TEXTED A FRIEND SAYING SHE WOULD RIDE HER BIKE. THE FBI IS INVOLVED AND I ASKED THE SPECIAL AGENT INVOLVED ABOUT THEIR INVOLVEMENT. SHE IS A MINOR SO WE TREAT THAT VERY SERIOUSLY. WE BROUGHT IN NATIONAL ASSETS THAT ARE COMING IN TODAY. IT WILL OR WILL HE -- IT IS IN SUPPORT OF THE WAUKESHA COUNTY POLICE DEPARTMENT. PERHAPS SOMEONE WHO MAY HAVE ADOPTED -- ABDUCTED HER. SHE MAY HAVE SIMPLY RUN AWAY. HE DID SAY THERE ARE NOW 18 WAUKESHA COUNTY SHERIFF DETECTIVES WHO ARE HELPING WITH THIS INVESTIGATION. THIS IS A VERY ACTIVE FBI IN MITIGATION AS WE HEAD INTO THE WEEKEND. WE ARE GOING TO HEAR MUCH MORE ABOUT THIS INVESTIGATION THROUGHOUT THE NIGHT. THANK YOU. THIS IS THE PICTURE OF THE BIKE THAT WAS FOUND. IT WAS FOUND NEAR WAUKESHA MEMORIAL HOSPITAL. WE WANT TO SHOW YOU ON THAT HERE. THE MAP SHOWS YOU THE DISTANCE BETWEEN HER HOME AND THE SCHOOL AND THE HOSPITAL. IT IS ABOUT EIGHT MILES TO THE EAST. THE DISCOVERY OF THIS BIKE AND ITS LOCATION RAISES A LOT MORE QUESTIONS. DETECTIVES CANNOT FIND ANYONE WHO SAY HOW IT GOT THERE. IT WAS FOUND RIGHT BY THOSE BUSHES RIGHT IN FRONT OF THE WAUKESHA MEMORIAL HOSPITAL. IT IS ONE MORE FRUSTRATE PEACE OF THIS INVESTIGATION -- PIECE OF THIS INVESTIGATION. THE BROADER GROUND EFFORTS THAT WERE THE FOREFRONT FOR THE SEARCH OF CAP INSTALLED WARM -- KATHYRN. THEY WERE ASKING IF ANYBODY IN THE NEIGHBORHOOD HAD SEEN HER. IS JUST THE FACT THAT THE BIKE HAD BEEN FOUND IN THE VICINITY -- THEY ARE WONDERING IF ANYBODY HAS SEEN ANYTHING. THE BIKE WAS FOUND UNDAMAGED HERE THURSDAY EVENING ACROSS THE STREET FROM WAUKESHA MEMORIAL HOSPITAL. WE FEEL THAT THE CITY IS AN IMPORTANT PLACE TO BE PAYING ATTENTION TO. THE BIKE IS SO FAR THE ONLY SOLIDLY THEY HAVE APPEARED WE ARE LOOKING AT A LOT OF VIDEOTAPE. SOME THE LEADS ARE NOT GAINING A LOT OF TRACTION BUT WE ARE TRYING TO PUT THINGS TOGETHER. WE DON'T HAVE A CAMERA POINTED AT THE PLACE WITH THE BIKE WAS. INVESTIGATORS WON'T SAVE ANY EVIDENCE WAS FOUND ON THE BIKE. THERE IS REASON TO THINK THAT SHE BROUGHT THE BIKE AS THERE IS TO THINK SHE DID NOT. WE DON'T KNOW. WE HAVE LEARNED THAT INVESTIGATORS EVEN TRAVELED TO MINNEAPOLIS TO MEET WITH CATHERINE'S ADULT SISTER THERE. THEY HAVE RULED THAT OUT HERE TO SHE SAID SHE DOES NOT KNOW WHERE SHE IS. SHE ALSO DO NOT PACK ANYTHING FROM THE HOME. THERE WAS NOTHING MISSING SO IF SHE WAS A RUNAWAY THEY SAY THIS WOULD BE AN UNUSUAL CASE. COMING UP, INVESTIGATORS HAVE A MESSAGE IF SHE CAN SOMEHOW HEAR THEM, THEY HAVE A MESSAGE DIRECTLY TO HER. WE WILL HEAR THAT TONIGHT AT 6 P.M. THANK YOU. THE STORY IS GETTING NATIONAL ATTENTION NOT JUST ONLINE BUT ON NETWORK NEWS. GOOD MORNING AMERICA TALKED ABOUT HER MS. -- HER DISAPPEARANCE. YOU CAN GO TO OUR FACEBOOK PAGE AND PRESS THE SHARE BUTTON FOR PICTURE. HER FRIENDS MAY ALSO SEE YOUR PICTURE. SHE DISAPPEARED TUESDAY MORNING. HER BIKE WAS FOUND EIGHT MILES FROM OUR HOME WHERE SHE NORMALLY RIDES. SHE IS 15 YEARS OLD, FIVE FEET SEVEN INCHES TALL. SHE WAS LAST SEEN WEARING JEANS, SWEATSHIRT, AND BRIGHT PINK NIKE SHOES.

Hillary Clinton's office said "nothing nefarious was at play" when the former secretary of state used her personal email address, rather than one provided by the State Department, during her four years as America's top diplomat.